1. Field of the Invention
This invention relates to an adjustable chassis for a scanner and particularly to an adjustable chassis with variable brightness for a charge coupled device to obtain an optimum image brightness to enhance scanning quality.
2. Description of the Prior Art
Scanner is a widely used peripheral device for a computer to capture and process document images. FIG. 1 shows a conventional scanner 1 which includes a chassis 12 for this purpose. FIG. 2 illustrates the detailed structure of the chassis in use. A document 2 is placed on a transparent document board 14 which is located above the chassis 12. The chassis 12 includes a light source 121, a reflection means 123, a lens 125 and a Charge Coupled Device (CCD) 127.
The light source 121 includes a lamp tube 1212 located in a curve-shaped chamber of a lamp holder 1214 for emitting light through a slot opening for scanning use. The length of the lamp tube 1212 is about the same of the width of the document board 14.
The reflection means 123 is to direct the image light of the document 2 to the lens 125. It generally includes at least one reflection mirror 1231.
The lens 125 is a condensing lens which receives image light from the reflection means 123 and forms a real image at its focal point.
The CCD 127 is located at the focal point of the lens 125 to receive and convert the image light to digital signals for a computer (not shown in the figures) to process.
It is generally known that the scanning quality of the scanner is highly depends on the brightness of the image light of the document 2 projecting upon the CCD 127. Brightness of the image light may be divided from 0 to 255 separated degrees with 0 represents least brightness and 255 represents most brightness. In general, brightness between 235 and 240 degree may result in best scanning quality. A chart in FIG. 2 shows relationship of document 2 brightness against the position of the document 2. The optimum brightness (i.e., between 235 and 240 degrees) may be obtained at a document displacement of about 5.5 mm.
However the chassis 12 usually has error more than 5.5 mm because of manufacturing problems and limitations. As a result, a conventional scanner rarely can provide the optimum brightness for the scanning document 2, and thus result in not desirable scanning quality.
The causes of this deficiency includes the following:
a. The quality of the lamp tube 1212 is not stable or consistent. The power rate of each lamp tube 1212 has different error when produced. The brightness which the document 2 receives from the lamp tube 1212 is thus also different.
b. The light projection angle of the lamp tube 1212 against the document 2 is different. Small projection angle produces greater brightness while large projection angle produces weaker brightness. Every scanner has slightly different projection angle due to human factor when assembling the light source 121 in the scanner 1. As a result, the brightness of the document 2 received is also different.
c. The reflection mirror 1231 has different reflecting angle resulting from human factor during assembly. This will also result in various brightness of image light reflecting by the reflection mirror 1231.
In order to resolve the brightness variance problem set forth above, there is a design as shown in FIG. 3, which includes a dimmer 322 in the light source 321. The dimmer 322 has a PLC (program logical circuit) to change the voltage of the light source 321 so that the brightness of the light source 321 may be changed from too high (such as the broken line C) or too low (such as the solid line A) to a desirable degree (dash line B). The addition of the dimmer 322 increases product complexity and cost. Furthermore it addresses only one of the factors mentioned above. Its effectiveness is thus not satisfactory.
In view of aforesaid disadvantages, it is therefore an object of this invention to provide a brightness adjustable chassis that includes a light source, a reflection means, a lens, a charge coupled device and a reflection mirror adjuster. While the construction of the chassis is largely like the conventional one set forth above, the reflection mirror adjuster may change the reflection angle of the reflection mirror so that the charge coupled device may received better brightness of light image to achieve better scanning quality.
It is another object of this invention to provide a brightness adjustable chassis that includes a light source, a reflection means, a lens, a charge coupled device and a lamp holder adjuster. The lamp holder adjuster may change the light projection angle of the light source upon the scanning document to obtain a desirable brightness for better scanning quality.
It is a further object of this invention to provide a brightness adjustable chassis that includes a light source, a reflection means, a lens, a charge coupled device and a light source adjuster. The light source adjuster may change the distance between the light source and the scanning document so that an optimum brightness may be obtained on the light image of the document for achieving better scanning quality.